Why Do RNA Viruses and Retroviruses Have Such High Rates

Why do RNA viruses and retroviruses have such high rates of mutation?

A) because RNA polymerase is an unstable enzyme
B) because RNA viruses and retroviruses initially cause host-cell mutations during replication
C) because RNA is more fragile than DNA and is therefore more likely to be damaged
D) because viral polymerases lack a proofreading mechanism
E) because viral RNA can form stem-and-loop structures